


Eggs

by doyouwanttohaveabadtime



Category: Undertale (Video Game)
Genre: Belly Expansion, Egg Laying, Other, Oviposition, Tentacles, slight praise kink
Language: English
Status: Completed
Published: 2015-11-16
Updated: 2015-11-17
Packaged: 2018-05-02 00:39:55
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 2,421
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5227247
Author URL: https://archiveofourown.org/users/doyouwanttohaveabadtime/pseuds/doyouwanttohaveabadtime
Summary: <blockquote class="userstuff">
              <p>Some good old oviposition fic for all the sinners out there.</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

> [coughs] Alrighty so I wrote another thing, this one’s way more self-indulgent and is obviously weirder so if it’s not your deal, I totally get it. Reader is of ambiguous gender with ambiguous genitalia, I tried my best on this so hopefully it came through!
> 
> Originally posted at my tumblr; doyouwanttohaveabadtime.tumblr.com.

After agreeing to help Gaster with an experiment, you wind up in his laboratory naked and spread out on an exam table. Tentacles lazily wrap around your body and the thickest appendage, bigger than all the others, rubs against you, slicking you up. "Alright, I’m ready,” you say nervously, cheeks burning.

Gaster rubs your leg in a comforting gesture. [DON’T FRET. IF YOU WANT ME TO STOP, I WILL. JUST SAY THE WORD.] He chuckles. [I WANT THIS TO FEEL GOOD FOR YOU. I’M SO OVERJOYED THAT YOU’RE PARTICIPATING, I CAN’T WAIT TO BE INSIDE OF YOU AGAIN.] 

You feel yourself flush even more as he praises you, and you squirm on the table. “Yeah, I can’t wait too,” you say truthfully. “Last time felt amazing.” 

Gaster hums at your response and the tentacle throbs against your body. A moment passes and it finally penetrates you slowly, filling you with ooze as it burrows inside. You feel it pulse and expand inside of you and when you look back up, you see a small bulge moving through the tendril. Soon, you feel something hard being pushed into you, and you whimper as you tighten up when it fully slides inside. Another egg slithers down the tentacle, squeezing into you with a squelch. You clench again, impulsively, but the tentacle opens you up again and deposits more ooze inside.

[IS THIS ALRIGHT?] Gaster asks, gently patting your stomach, admiring the small bump that’s already started to develop. [YOU’RE DOING SPECTACULARLY. YOU’RE THE BEST ASSISTANT.] 

His words give you a momentary morale boost but you yelp as another egg is laid, and then another right after it. Slime seeps out of your entrance. You notice that your belly is starting to show the effects of the brood inside of you. A particularly large mass shoots toward you and you can’t help but cry out loudly as it presses against you. The tentacle pumps you full of ooze, attempting to get the egg inside. “Gaster, i-it’s too big!” you whine. “It’s not going to fit!”

As you squeeze your eyes shut, you feel the tentacles forcefully shift your position and lift your legs higher, giving the biggest tentacle better leverage. It surges and stretches and the egg finally enters you with a plop. A gush of liquid follows it, and another egg slips past your opening. You vaguely wonder how many eggs Gaster intends on filling you with.

The tentacle shows no sign of stopping soon, and it keeps pumping egg after egg into your swollen body. By now, you feel incredibly full, and your stomach is stretched out more than you’ve ever experienced before. You’ve lost track of time and you feel completely exhausted.

Gaster’s voice brings you out of your haze. [JUST ONE MORE EGG, AND THEN YOU’RE DONE.] he states, clasping his hands together. You can see that the last egg shooting down the tentacle is much larger than any of the others. You whimper as it makes contact, but it slides in easier than you thought. You wonder how stretched out you are. A final slurry of fluids enters you, and you feel the tentacle pull out. You immediately clench up, but a wave of ooze drips onto the bed and floor. You try to steady your breathing.

The tentacles unwrap from your body and Gaster steps forward, dwarfing your body. His grin is the largest you’ve seen so far. [THAT WAS TRULY AMAZING. YOU DID SO WELL, HUMAN.] he exclaims. He leans down and nuzzles your body. You let out a breathy moan. 

[BUT, I’M NOT DONE YET.] he says eagerly, running his hands across your bloated stomach. [I WANT TO DO SOMETHING FOR YOU SINCE YOU PERFORMED SO PERFECTLY.] Before you know what’s happening, Gaster wraps his arms around you, gently scooping you up. [THOUGH, SOMEWHERE ELSE, LESS CLINICAL.] You feel the eggs jostle in your belly as he hoists you off the table. Vaguely, you can see one of his phantom hands grabbing a cup of something on the way out. 

He makes his way down a corridor that eventually leads to a small bedroom, holding you close to his body. Despite the swollen heavy feeling in your stomach, this experiment hadn’t been too bad. All the positive attention was making you feel warm and content. Gaster gently sets you down on the bed inside the room and closes the door behind him. By now, the eggs were starting to press against you, causing quite a bit more pressure than you had experienced before. You make a low noise as a particularly large one causes a cramp, and you squirm and whimper in discomfort. 

[OPEN.] Gaster says, putting the cup he brought with him to your lips. You obey and drink it down, grimacing at the bitter taste. [IT’S A LIQUID PAINKILLER, IT WILL HELP WITH THE CONTRACTIONS.] You give a small smile of appreciation and lift your arms up, reaching out towards him. Gaster slides onto the bed and positions himself above, his form completely enveloping you. You feel safe and protected in his embrace. 

[MY WONDERFUL HUMAN.] he moans, rutting his torso against your body. You tug him closer and his tongue slides out of his mouth, slicking your lips and entering yours. You groan and kiss him as best as you can, rubbing your tongue against his. Soon, you feel his erection pressed against you and as you reach down and start stroking it, Gaster shudders, bucking himself into your grasp. Fluid leaks out of the tip and covers your hand. 

“I want to make love to you,” you mumble bashfully as he slides a hand through your hair. Whatever liquid he gave you worked fast--you certainly felt better than you did before.

[AND I AS WELL.] he replies before slowly sliding his cock into you. The added lubrication allows him to enter you easily, and he lets you adjust before starting to pump into you hard and fast. You feel the tip of his arousal brush against the last of the eggs every time he pulls out and thrusts back in, and the thought of it somehow makes you even more turned on. [YOU’RE PERFECT.] he groans, and you choke on a moan as he rubs up against you just the way you like it. You can feel the phantom hands touch you everywhere, massaging your belly and gripping your thighs. A pair wraps around your neck and squeezes feather light. Your tongue lolls out of your mouth as you try to match his pace.

“H-harder!” you exclaim as you grip the sheets and buck your hips. Gaster hoists your legs up like before and pounds into you rough and fast. You’re completely at your limit and you shriek and shudder as your orgasm washes over you. Gaster whines and thrusts one last time, hard enough to jostle the eggs, before releasing load after load of sticky cum inside you. Your body feels limp and exhausted, numb from being fucked so much. 

Some time later, minutes, hours maybe, you can’t tell anymore, Gaster pulls out and a long sticky stream of ooze plops onto the bed. The eggs shift inside of you uncomfortably. The phantom hands ghost away and he brings his corporeal hands up to your stomach again, rubbing the taut flesh, feeling the individual bump that each egg makes. It’s a weird, almost arousing sensation and you shiver from his touch. 

[YOU’LL MAKE SUCH A GOOD HOST.] he utters softly. [THE EGGS NEED A WARM PLACE TO INCUBATE, AND YOUR BODY WILL DO WONDERFULLY.] 

When you look up, Gaster’s face reveals what you think might be his version of embarrassment, cheeks a dark shade of grey. It’s completely and utterly endearing. You laugh in response and tug him closer, gently kissing his cheek. “I-I’m flattered you chose me,” you mumble against his face. “I like being your assistant.”

He shifts his form to embrace you from the side, and you feel that same warmth again as he completely surrounds your body. With a yawn, you attempt to roll over before determining that was a terrible idea, so you settle for just scooting closer to him. He laces his fingers in between yours and rubs your palm soothingly. A sense of calm washes over you and you close your eyes. “Will you stay here with me?” you ask quietly. 

[OF COURSE.] he responds, and you drift off to sleep.


	2. Chapter 2

**Notes for the Chapter:**

> Reader is still of ambiguous gender with ambiguous genitalia. There’s no sex in this one, sorry! I had enough trouble writing the egg laying part haha.
> 
> Originally posted at my tumblr; doyouwanttohaveabadtime.tumblr.com.

You weren’t sure how long the eggs would incubate for. Gaster gave you a brief approximation and explained that since he hadn’t done this type of experiment in a while, he unfortunately wasn’t completely positive on the numbers. All you knew was that not even a day later after he laid the eggs inside of you, you could barely tug your shirt over your stomach and your pants refused to zip or button, so you’d taken to wearing sweatpants instead. 

Gaster had been taking good care of you at least, feeding you and bringing you blankets and reading material and a heating pad. He doted on you nearly all the time when he wasn’t working on his research and while you weren’t great at showing it, you secretly loved it. You loved every time he massaged your poor bloated stomach and you loved how it felt to sleep with him at night and you loved being his one and only research assistant. That made all the pain worth it.

On the third day, you developed a twinge in your lower abdomen. It started out annoying but eventually progressed into uncomfortable pain that radiated constantly. Every time you touched your sensitive belly it made you wince. You managed to roll out of bed and stumble into the laboratory where Gaster was working on some machine, clutching your rumbling stomach all the while. “Gaster, uh, I think something’s wrong,” you grunt, feeling the eggs shift inside of you. You drop to the floor on your hands and knees as tremors wrack your body. 

You feel his presence before you actually see him, his form quickly ghosting around you. He rubs your back patiently and inspects your belly. [AH YES, I’M SURE YOU’RE READY TO LAY RIGHT NOW.] he says calmly, tugging your pants and underwear down. You quickly kick them off and whine as another wave of pain hits. You can already feel the first egg press desperately at your entrance and you lower your head in embarrassment. [DON’T WORRY, I’LL TAKE GOOD CARE OF YOU.] Gaster promises. 

“I-it’s coming out,” you moan. You shift your position and the egg squeezes out with an audible pop, covered in dark ooze. You can barely catch your breath before the next one slips into place and slides out of you easily. Gritting your teeth, you press your forehead to the cold floor, trying to relieve the pressure in your belly. You feel the third egg strain against you and judging from the way your entrance widens, you think it’s one of the larger ones. 

“T-this one is really big,” you say through clenched teeth. Your vision is blurry and the pain spasms keep getting worse the more you push. With one pair of hands, Gaster massages your belly, another separates your legs, and yet another spreads your entrance gently, encouraging the egg to crown. With a shaky gasp, you push one last hard time and the egg falls out, plopping wetly on the floor. A stream of slime drips out of you.

[ONLY SEVERAL MORE TO GO.] Gaster comments, rubbing your shoulders tenderly. [YOU’RE DOING SO WELL, MY HUMAN.] You readjust yourself quickly to get more comfortable before you feel the next few eggs shift down. They slip out without any trouble and join the others on the floor with a squelch. A wave of nausea hits and you swallow hard. There’s a puddle of drool on the floor.

Two more eggs later, and you vaguely think this is the last one. Your belly has thinned out considerably and the dull pain has merely turned into an annoying pressure in your gut. You brace yourself again and the last egg stretches you wide. With one last shaky push, it finally lands on the floor followed by a splash of fluid, joining the rest of the slime-covered brood. 

With all your energy drained, you collapse on the floor and wipe your damp forehead with the back of your hand. Now that all the eggs were out, you felt significantly better, though you were also pretty exhausted from all that work. You yawn and blink sleepily before deciding you felt well enough to slowly sit up again and look at your clutch. 

[LOOK.] Gaster muses as he gathers the eggs in his sticky embrace. There were 8 of them in total, dark as the night sky and much bigger than a chicken’s, though 2 were considerably larger in size. Despite the weirdness of it all, they were quite pretty. Gaster’s wide smile nearly cracks his face open as he holds them out to you. You take one in your hands, palming it gently.

“Wow,” you say with a smile on your face. “All that work for these, huh? That’s....really something.” A blush spreads across your cheeks as you try to think of the words to say. You eventually settle for handing the egg back to Gaster before hugging him gently. As you nuzzle your head into his body, he wraps his arms around your form, enveloping you. A gentle static hum emanates from him, and the vibrations tickle your face. You chuckle and look up. “Can I take a nap? This really tired me out.” You try to stifle a yawn, though it slips out anyways.

[OF COURSE. YOU DESERVE A NICE REST.] he says eagerly, effortlessly picking you up and cradling you gently in his arms. You coo happily as he carries you back to your bed and tucks you in. [REGRETFULLY, I NEED TO LOCATE AN INCUBATION METHOD FOR THE EGGS, BUT I’LL BE BACK LATER.] he promises. You make a dissatisfied noise but it’s more playful than anything. You honestly feel too tired to do much but sleep.

“That’s fine,” you mumble sleepily, burrowing under the blankets. “I’m pretty exhausted from all that work anyways.” 

As you make yourself comfortable in the bed, you hesitate for a moment before whispering a quiet “I love you,” and as you drift off into blissful slumber, you swear you hear Gaster echoing it back to you too.


End file.
